1. Field of the Invention
The present invention relates to a projection exposure apparatus and, more particularly, to a scan type projection exposure apparatus used to manufacture semiconductor integrated circuits and liquid crystal devices.
2. Related Background Art
Many conventional apparatuses of this type have correction functions for imaging characteristics because the apparatuses need to maintain high imaging characteristics. Factors which cause the imaging characteristics to vary are changes in external environment such as atmospheric pressure and temperature, and slight absorption of exposure light by a projection optical system. With regard to changes in environment, the atmospheric pressure and the like are monitored by sensors, and correction is performed in accordance with the detection values, as disclosed in, e.g., U.S. Pat. No. 4,687,322. With regard to absorption of exposure light, light energy incident on a projection optical system is measured, and a change in imaging characteristic owing to absorption of exposure light is calculated on the basis of the measurement value, thereby performing correction, as disclosed in, e.g., U.S. Pat. No. 4,666,273. In this known method, light energy incident on the projection optical system through a mask is detected by, e.g., a photoelectric sensor arranged on a substrate stage. In addition to light energy for projection exposure, which is incident from the mask side, light energy is incident on the projection optical system after it is reflected by a photosensitive substrate. This light energy also changes the imaging characteristics of the projection optical system depending on the intensity. With regard to such light energy, for example, as disclosed in U.S. Pat. No. 4,780,747, light reflected by a photosensitive substrate is measured by a photoelectric sensor arranged in an illumination optical system. The sensor receives the light through a projection optical system and a mask, and a total change in imaging characteristic is calculated in consideration of a change in imaging characteristic owing to this reflected light energy. In this method, light reflected by an optical member, a mask pattern, and the like is incident on the photoelectric sensor in the illumination optical system together with light reflected by the substrate. For this reason, a plurality of reference reflecting surfaces having different known reflectances are set on a substrate stage, and the ratio of the respective outputs from the photoelectric sensor, which correspond to the reference reflecting surfaces, is obtained in advance. The reflectance (more accurately, reflection intensity) of the photosensitive substrate is obtained on the basis of this ratio. As described above, since light reflected by a mask pattern is superposed on light reflected by a photosensitive substrate, sensor outputs corresponding to a plurality of reference reflecting surfaces must be obtained every time a mask is replaced. Alternatively, sensor outputs must be measured and registered in advance.
Conventionally, the amount of change in imagining characteristic owing to absorption of exposure light is obtained to perform correction by the above-described methods.
The above conventional scheme has been developed on the basis of a scheme of projecting/exposing the entire mask pattern on a photosensitive substrate (called a batch exposure scheme or a full field scheme). Recently, however, a so-called scan exposure scheme has been developed, in which exposure is performed by illuminating a portion of a pattern area on a mask with a slit-like beam while moving the mask and a photosensitive substrate relative to each other. In this scheme, since the illumination area on a mask is smaller than that in the batch exposure scheme, the amount of image distortion or illuminance irregularity is small. Furthermore, no limitations are imposed on the field size of a projection optical system in the scan direction, and hence large-area exposure can be performed.
In a scan type exposure apparatus, however, energy incident on the projection optical system changes while a mask is scanned with respect to a slit-like illumination beam. For example, such a change occurs because the area of a light-shielding portion (a chromium layer of a pattern) formed on a mask changes in accordance with the position of a slit illumination area on the mask, and hence the amount of energy incident on the projection optical system during a scan exposure operation changes.
In addition, the amount of light reflected by a mask pattern changes in accordance with the position of a mask. Therefore, the detection precision with respect to the amount of energy which is reflected by a photosensitive substrate and incident on the projection optical system inevitably deteriorates in the conventional scheme.
For the above-described reasons, in the conventional scheme, correction based on an accurate amount of change in imaging characteristic owing to absorption of exposure light cannot be performed.